This invention relates to capacitive discharge (CD) ignition systems for internal combustion (IC) engines, and more particularly to improved CD ignitions with much higher efficiencies and much higher spark firing rates than achievable before for a given level and type of delivered spark energy. The invention is especially useful for the very efficient and rapid delivery of high energy spark discharges of the flow-resistant type which are preferred in advanced high efficiency IC engines with high in-cylinder airflows. The invention applies to both single coil distributor type ignition systems as well as to more modern one-coil-per-plug distributorless type ignition. In the case of the distributor version, the high efficiency and high spark firing rate make the system especially useful in high speed eight cylinder (V-8) engines operating at speeds up to and above 9,000 RPM and providing the more useful flow resistant, single polarity, triangular, arc discharge mode type spark with minimum heat dissipation. In the case of one-coil-per-plug ignition, the high efficiency of the system allows for delivery of more of the limited available energy associated with smaller ignition coils and/or lower electrical energy generating systems, as in a flying magnet system found in small engine applications. The invention relies, in part, on the use of ignition coils with improved side-by-side windings and with improved silicon-iron laminated cores for achieving the higher efficiencies, and in part on the use of IGBT and FET switches and high efficiency diodes for controlling and shaping the discharge of the energy storage capacitors (also referred to hereinafter as xe2x80x9cdischarge capacitorsxe2x80x9d) as well as the primary and secondary winding currents to insure good operation throughout a wide range of speeds, from cranking to very high speeds or high firing rates.
Current capacitive discharge (CD) ignition systems are very inefficient, with typically 15% to 25% efficiency, and deliver typically only 20 to 30 millijoules (mJ) of spark energy per single spark pulse (into an industry standard 800 volt Zener load). On the other hand, evidence points to a requirement of over 100 mJ of spark energy for best engine performance of a standard automobile engine. In addition, CD ignition coils are typically wound with concentric primary and secondary windings to give relatively low leakage inductance for a given number of primary wire turns, which contributes to the low efficiency of the ignition system, versus the three times and higher efficiency of 60% to 70% achieved in the present invention.
Current CD systems also typically use silicon control rectifiers (SCRs) which discharge all the capacitively stored energy upon ignition firing. This results in slower recharging of the discharge capacitors to a potentially lower energy and peak voltage, as well as poorer use of the power source. This can be a particular problem when lower energy is available for delivery to the capacitor, as in the case of engines running at very high speeds (less charging time) or flying magnet systems under engine cranking conditions. On the other hand, the present system uses switches that can be turned off, preferably insulated gate bipolar transistors (IGBTs), and a discharge circuit design and control which allows for the capacitor charging during ignition spark firing.
Modern ignition coils use laminations which are butt-welded and have mounting holes punched in the laminations. Such designs reduce the coil discharge efficiency and increase coil heat dissipation. Moreover, they use relatively thick laminations, i.e. 14 mil (thousands of an inch) or thicker, which results in lower coil efficiency when applied to the present, more optimal, preferred side-by-side winding coil topology which is more sensitive to certain aspects of lamination design, as was discovered, requiring alternative designs for the laminations, geometry, and mounting.
In the present application, segmented secondary coil windings are provided to limit coil output capacitance Cs (to help insure voltage doubling), as well as inductive suppressor wire to reduce electromagnetic interference (EMI), where applicable. However, to improve the spark""s ignitability, capacitive spark plug boots, or improved spark plugs with built-in capacitance, are preferred for the present application.
In the case of high speed engines with single coil distributor ignitions, high circuit efficiency along with use of the preferred triangular, versus sinusoidal, primary and secondary winding spark current distribution leads to problems at high engine speeds when a coil may be fired well above 200 Hertz (Hz). This problem occurs because of the imperfect coupling between the primary and secondary windings (k less than 1), resulting in a residual primary current after the secondary current has dropped to zero. The residual primary current then decays at a much lower rate dictated by the ignition coil primary circuit losses alone so that at high coil firing rates there may be a non-zero primary current when the ignition refires, reducing the secondary spark current at high speeds. While this problem may be largely off-set by designing the discharge circuit for the sinusoidal current distribution, this has several drawbacks, including and not limited to not allowing charging of the discharge capacitor during spark firing. In the present invention is disclosed improved and optimized methods of handling this problem.
In the automotive case where battery power is used (12, 24, or future 42 volt), the power converters which are used to step-up the voltage to the typical 200 to 600 volts are generally inefficient and electrically noisy, with efficiencies between 35% and 70%, and up to 85% in practice in my U.S. Pat. No. 5,558,071. In the present application is disclosed improvements to increase power converter efficiency to between 90% and 94%, important in minimizing heat dissipation in the high temperature engine environment, especially where small, lightweight packaging of the parts is preferred.
By careful and innovative design of the entire system, from the power stage, to the discharge circuit, to the ignition coil, to the spark plug wires and spark plug itself, one can achieve a very high efficiency and a high spark discharge energy. In the case of a single coil distributor ignition system one can have a more optimized system for very high speeds with much higher energy density (spark energy per unit weight) with minimum heat dissipation, and in the case of one-coil-per-plug ignition systems one can have very small, low cost parts, producing high spark energy very efficiently.
The system of the present invention is applicable to single coil distributor and one-coil-per-plug distributorless CD ignition systems. The system uses controllable coil primary winding circuit main switch means Si (S for distributor systems) which can be turned-off prior to the discharge of the energy storage capacitor, and diode means Di (D for distributor systems) shunting the coil primary winding of high efficiency coils with side-by-side windings producing an essentially triangular distribution of primary current Ip and secondary current Is. The system is designed to provide the highest efficiency as a complete system as well as in terms of individual parts and sub-systems. It delivers maximum spark energy for a given stored energy, produces low component and system heat dissipation, and provides the most rapid and efficient recharging of the discharge capacitors, especially at very high switching speeds as occurs in single coil distributor ignition systems found in high speed multi-cylinder engines.
In a preferred embodiment, the discharge capacitor is not fully discharged by the main discharge switch S or Si, which is preferably an IGBT, which operates to leave a significant voltage on the capacitor, e.g. 20% to 40% of the initial voltage, or approximately 130 volts for the high firing rate distributor ignition case of preferred capacitor voltage Vc of 360 volts, which allows for quicker restart of the power converter and more rapid recharge of the capacitor, especially at high engine speeds in a V-8 engine where there is not much time for charging.
In another preferred embodiment, especially useful for single coil distributor ignition systems operating with a preferred high efficiency flyback power converter, use is made of an additional second switch SD in the discharge circuit in series with the shunt diode D, which is closed during most of the spark firing and then modulated 180xc2x0 out-of-phase with the power converter switch SPC after the spark current Is has neared zero current or after it has reached zero current. In doing this: 1) the residual primary current is diverted and diminished rapidly instead of building up in the coil primary winding to lead to core saturation; 2) the residual primary current Ip is diverted back to the battery for best efficiency; and 3) the diversion of the residual primary winding energy occurs in a way that presents a low voltage drop during the charging stage of the discharge capacitor by the power converter, i.e. when the power converter switch SPC is turned off, maximizing power converter efficiency and output power with coil primary residual energy diversion. In this way, the ignition coil may be fired at a very high rate with the highest possible efficiency, even with the preferred triangular current distribution. Timing for turning off switch SD and beginning its out-of-phase modulation is obtained preferably by monitoring the spark current Is with a sense resistor and transistor, such that when the current Is is small, say {fraction (1/10)}th its peak, switch SD may be turned off and modulated. However, other means of turning off switch SD may be used, especially if the spark firing duration is known at high speeds.
The ability to provide very rapid firing of an ignition coil without limiting the coil energy or depending on coil resistance to damp the residual primary current, allows for the design of very high energy coils with the highest efficiency ever attained. Such designs are achieved with coils with side-by-side windings. They are further improved by allocating greater winding length, i.e. approximately ⅓ the total winding length to the primary winding, and using Litz wire when several layers of heavy gauge wire are required to minimize coil AC resistance. The coil efficiency is further increased by another factor relating to the recognition of the non-symmetrical nature of the magnetic flux in the magnetic core and the resulting magnetic leakage flux that cuts across the winding window on the primary winding side of the core, requiring thin, preferably 6 mil laminations, or other low loss magnetic core material such as powder iron, in the half of the core associated with the primary winding. Use of high leakage inductance Lpe of the coil, typically in the 200 to 500 microhenry (uH) range, resulting in lower frequency operation, and the use of the preferred triangular current distribution with large direct current (DC) component, also increase coil efficiency by minimizing the high frequency loss effect of the leakage flux for a given thickness of lamination in the primary winding half of the core.
Typically used heat sink material, such as aluminum extrusions surrounding the coil, absorb significant coil energy in the present side-by-side winding case through eddy currents produced by the leakage magnetic field. In the present system, heat sinks are designed to minimize eddy current losses by confining them to the coil end sections, or by using non-electrical but high thermally conductive, cast, aluminum powder based heat sink material and coil mounting parts. The coil and housing structure is made to provide good heat transfer from the windings and central coil regions to the outside of the coil. For preferred laminated silicon-iron cores, preferably square (or rectangular) winding bobbins are used with square center hole and winding with minimum wall thickness without electrical breakdown, especially on the primary winding side. If round bobbins are used, laminations with several center leg widths are used to minimize the paths between the outside surface of the center leg sections and inside of the windings and to maximize magnetic core area. Also, hybrid cores can be used with powder iron in the primary winding section of the coil and laminations in the other half.
For the coil parameters, approximately 50 turns of primary winding Np are preferred, i.e. 40 to 60 turns, and a turns ratio N of approximately 65 is used for 400 volt rating discharge capacitors. This gives a leakage inductance Lpe in the range of 200 to 500 uH, depending on the specific coil design for the preferred side-by-side winding. Peak spark current Is is typically in the range of 200 to 800 milliamps (ma), depending on application, and preferably in the 400 to 600 ma range for good spark flow-resistance with acceptable spark plug erosion. For the single, large coil, distributor application, preferably 13 to 15 equivalent AWG (American Wire Gauge) Litz wire is used in a four to eight layer primary winding, preferably six layer, to provide a good fill of the primary winding section or bay, with 30 to 33 AWG magnet wire for the secondary winding. For the smaller, one-coil-per-plug application, preferably 15 to 18 AWG magnet wire is used (preferably in six layers) and selected to give primary winding AC resistance close to the DC resistance, and preferably 33 to 36 AWG magnet wire is used for the secondary winding. In both cases, preferably the winding ends are on the same side of the coil with the high voltage tower on one end and the primary wire winding ends and secondary start lead on the other end, for ease of manufacture and use. The larger, distributor ignition, single coil is preferably cast in a mold with the outside surfaces of the laminations exposed to the air for best cooling. The distributorless ignition coils may be cast similarly, or in a housing, as they require much less cooling. As used herein, the term xe2x80x9capproximatelyxe2x80x9d means within xc2x120% of the term it qualifies. When there is no qualification on the value of a parameter, it shall be taken to mean the value xc2x110%.
In the single coil distributor designs shown, the discharge topology preferred is Type II, in which the discharge capacitor is in series with the coil primary, one end of which is grounded. The shunt diode placed across the primary winding is the path through which the capacitor is charged. In this topology, the initial negative voltage on the primary winding side of the capacitor can be used as a signal to control turn-off of the main discharge switch S/Si for partial discharge of the capacitor. Type I topology, used in the one-coil-per-plug distributorless application, can be used in the distributor multi-cylinder version, where the capacitor is across the power converter output, and the main switch is in series with the primary winding to ground.
In the case where high spark current is desired, e.g. one amp peak spark current for drag racing, a coil design using concentric windings but open end E-lamination may be used to provide the required low leakage inductance Lpe of approximately 80 uH, primary inductance Lp about ten times greater than Lpe, achieved with approximately 60 turns of primary winding. The core open end allows for easier and cheaper assembly of the coil, disclosed in my patent application PCT/US96/19898 on inductive ignition. Such designs will not be discussed in this application which emphasizes high efficiency achieved with the use of coils with side-by-side windings and by other means.
For automotive applications, to maximize circuit efficiency and minimize heat dissipation, the flyback converter disclosed in the cited patent and patent application is improved by lowering its frequency of operation to about 20 kHz, i.e. 12 to 28 kHz, where the term xe2x80x9caboutxe2x80x9d means within xc2x140% of the term it qualifies, to minimize switching losses of the power switch PCS, core and ultra-fast output diode. The power converter transformer Tr is designed with two or three layers of secondary winding (with low AC losses at the operating frequency) for lower transformer losses and higher secondary winding capacitance. Single layer primary winding is used with Litz wire. For a two layer secondary, the primary may be located between the layers to minimize leakage, known to those versed in the art. The power converter snubber is designed such that after power converter switch PCS opening, the snubber capacitor voltage decays to a voltage equal to the secondary voltage transformed to the primary side, and not to zero. In addition, a small high voltage capacitor may be placed on the output of the converter transformer Tr to reduce the ultra-fast output diode switching losses.
In this application preferably capacitive boots for the spark plugs or capacitive spark plugs are used, with inductive, low resistance wire under 20 ohms/foot if practical to reduce EMI without spark energy loss and without eliminating the capacitive or breakdown spark. High capacitance of the spark plug, e.g. 30 to 60 picofarads (pF), may be achieved by using high dielectric constant insulator material, e.g. an aluminum zirconia mix, with metallic coating on the insulator in the capacitance region.
It is a principal object of the present invention to provide CD ignition systems of both the distributor and distributorless type that have the highest possible efficiency, 60% to 70% battery-to-spark efficiency (using an industry standard 800 volt Zener as the spark-gap load), and to accomplish this by the choice of components and by the design of new components, sub-systems, and the complete system as a whole made up of power converter, discharge circuit, ignition coil, spark plug wire, and spark plugs.
It is another object of the present invention to use the high efficiency design of the present invention to provide a high energy spark of the triangular distribution type operating in the low arc discharge mode of 200 ma to 800 ma to provide the most effective ignition of an air-fuel mixture over the entire range of engine speeds and loads and engine types, all of which are accomplished by the present invention. This includes using the flow resistant features of the spark to advantage and including a high capacitance or breakdown spark with minimum EMI achieved by using capacitive spark plugs or boots in conjunction with the present system, to give the most effective ignition under all possible conditions.
Other features and objects of the invention will be apparent from the following detailed description of preferred embodiments taken in conjunction with the accompanying drawings.